The present invention relates to extreme ultraviolet (EUV) mirrors for EUV lithography. More particularly, it relates to methods for manufacturing EUV mirrors.
Extreme ultraviolet (EUV) lithography (EUVL) is a relatively new form of lithography. EUVL uses extreme ultraviolet (EUV, also called soft X-ray) radiation with a wavelength in the range of 10 to 14 nanometers (nm) to perform the imaging. Up to now, optical lithography has been the lithographic technique of choice in the high-volume manufacture of integrated circuits (IC). The constant improvement in optical lithography has allowed it to remain the semiconductor industry""s workhorse through the 100 nm or smaller generation of devices. However, to pack even higher density circuits into IC chips, new technologies (Next-Generation Lithographies, NGL) will be required. EUVL is one NGL technology vying to become the successor to optical lithography.
In many respects, EUVL is similar to the optical lithography. For example, as illustrated in FIG. 1, the basic optical design for an EUVL system is similar to that of an optical lithographic system. It comprises a light source 1, a condenser 2, a mask (reticle) 4 on a mask stage 5, an optical system 6, and a wafer 7 on a wafer stage 8. Both EUV and optical lithographies use optical systems (cameras) to project images on the masks onto substrates which comprise silicon wafers coated with photo resists. However, the apparent similarity stops here. Because EUV is strongly absorbed by virtually all materials, EUV imaging must be performed in vacuum, which is achieved by enclosing the system in a chamber 3. In addition, the chamber 3 might be further partitioned into different compartments 10 and 20, which have their own vacuum systems. Because EUV is absorbed by most materials, there are no suitable lenses to focus and project an image on mask 4 onto a substrate (wafer) 7. As a result, it is necessary to operate EUVL in a reflective mode instead of a transmissive mode. In the reflective mode, light is reflected from mirrors (not shown; inside the optical system 6), instead of being transmitted through lenses. Even with reflective optics, there are not many materials capable of reflecting EUV. In order to achieve reasonable reflectivities at near normal incidence (i.e., an incident beam landing on the surface of a mirror at an angle close to normal to the surface), the surface of a mirror is typically coated with multilayer, thin-film coatings. These multilayer, thin-film coatings reflect EUV in a phenomenon known as distributed Bragg reflection.
The multilayer coatings for the reflective surfaces in EUVL imaging system consist of a large number of alternating layers of materials having dissimilar EUV optical constants. These multilayers provide a resonant reflectivity when the period of the layers is approximately xcex/2, where xcex is the wavelength. The most promising EUV multilayers are coatings of alternating layers of molybdenuum (Mo) and silicon (Si). These layers are deposited with magnetron sputtering. Each layer of Mo or Si is coated to a thickness of xcex/4 of the EUV light so that it will have a periodicity of xcex/2. In this type of reflector, a small portion of the incident light is reflected from each silicon surface. The thickness of the layers causes the reflected light waves to interfere constructively. The more layers there are, the more light will be reflected. However, imperfections in the surface coating will eventually diminish the reflectivity return of more coatings. Currently, most mirrors in EUVL systems have around 40 alternating layer pairs of Mo:Si. Furthermore, most of these Mo:Si multilayers are optimized to function best with wavelengths at around 13.4 nm, which is the wavelength of a typical laser plasma light source.
A typical EUVL optical system or camera (see 6 in FIG. 1) consists of several mirrors (e.g., a four-mirror ring-field camera shown in FIG. 2). The mirrors that comprise the camera must have a very high degree of perfection in surface figure and surface finish in order to achieve diffraction limited imaging. It is predicted that the surface figure (basic shape) of each mirror must be accurate to 0.25 nm rms (root mean square) deviation, or better. In addition to surface figure, stringent requirement must also be placed on the finish of the surfaces. The challenge for a fabricator of optics for EUVL is to achieve the desired levels of surface figure accuracy and surface finish simultaneously.
FIG. 2 illustrates a typical prior art four-mirror optical system for EUVL application. Such an optical system is used to project and reduce an image from a mask onto a wafer. The reduction achieved by the optical system permits the printing of a image smaller than that on the mask onto a wafer. The projection operation is typically carried out in a step-and-scan process. In a step-and-scan operation, a light beam from a light source (see 1 and 2 in FIG. 1) is used to scan the image on the mask. The light beam L reflected from the mask is further reflected by four mirrors M1, M2, M3 and M4 in succession to project and reduce the image from the mask onto the wafer.
The high degree of precision in figure and finish required for EUVL imaging requires that the mirrors should be substantially invariant to environmental changes, e.g., temperature changes. In order to afford better thermal management, it is preferred that these mirrors be made of light weight materials with very low coefficients of thermal expansion (CTE). One promising material for such application is a ultra low expansion glass material, such as the ULE(trademark) glass from Corning, Inc.
ULE(trademark) glass has a CTE of about 0xc2x130 ppb/xc2x0 C. over the temperature range of 5 to 35xc2x0 C. The CTE in ULE(trademark) glass is a function of the titanium concentration. ULE(trademark) glass typically contains about 6 to 8 wt. % of TiO2. Compositions containing about 7 wt. % TiO2 have near zero CTE. In addition to having extremely low thermal expansion, ULE(trademark) glass is also unique in that it hAs no crystalline phase. In other words, ULE(trademark) glass is completely amorphous. ULE(trademark) glass is a high temperature glass which makes it unsuitable for manufacturing by conventional means. Instead of being poured, it is fabricated by a flame hydrolysis fused glass process which is similar in scope to chemical vapor deposition. In the flame hydrolysis process, high purity precursors of titanium oxide and silicon oxide are injected into flames to form deposit onto the surface of the growing glass. The process minimizes impurities such as sodium and other alkali or alkaline earth metals.
Due to its unique process of formation, ULE(trademark) glass is formed in layer deposits. This means ULE(trademark) glass inherently has striae, though these striae are not apparent and do not affect most applications. Although ULE(trademark) glass has been polished to 0.5 xc3x85 rms (root mean square) surface roughness, the striae may present problems for stringent applications like EUV mirrors. For example, it can create a mid frequency surface structure that would cause image degradation in mirrors used in the projection systems for EUV microlithography.
FIG. 3A illustrates a piece of ULE(trademark) glass 31, in which striae 32 are shown. When the striae are oriented perpendicular to the axis of a mirror and that mirror is subsequently ground and polished to a concave or convex shape, the striae planes are cut across. FIG. 3B shows a cylinder of ULE(trademark) glass 33 with its top ground to give a convex surface. It is apparent that different layers of striae planes are cut across, leaving approximately concentric circles of striae edges 34. This is not a problem for applications where the source light is in the range of visible to infrared. However, in EUV lithography, which uses lights with wavelengths around 13 nanometer (nm), these striae edges 34 may manifest themselves as small ridges. These can cause aberrations which would degrade any images projected within the optical train. Although this problem is illustrated with ULE(trademark) glass, the problem is not unique to this glass. The same problem due to striae will be encountered in any material that is prepared by gradual deposition of newly formed material onto the materials, like the flame hydrolysis process in the formation of ULE(trademark) glass. Such materials, which inherently have striae, will be generally referred to as the ULE(trademark) glass-like materials herein.
To make good use of the ULE(trademark) glass-like materials, it is desirable to have methods that can reduce the striae effect in the manufacturing of EUV mirrors of ultra low expansion materials.
Embodiments of the invention relate to methods for manufacturing mirrors for use in EUV lithography. Some embodiments include sagging a plate of a glass material to produce a mirror blank; and polishing a top face of the mirror blank to produce a polished mirror. Other embodiments include grinding a top face of a piece of a glass material; sagging a plate of the glass material over the top face of the piece to produce a mirror blank; and polishing a top face of the mirror blank to produce a polished mirror. The glass material may be ultra low expansion glass material with a coefficient of thermal expansion of no more than 30 parts per billion per degree Celsius in a temperature range of 5 to 35xc2x0 C.
Other aspects and advantages of the invention will be apparent from the accompanying descriptions and drawings.